For starters, a little thought...since both sine and cosine are, at most, unity (1), we would have trouble finding a solution if one of them managed to be negative. (Why is that?) This means we need to look ONLY where both not negative.

After saying that, you can substitute from the Pythagorean Identity, . Remember, .

Substitute into the original equation, rearrange a little, square things, and you should be able to find a quadratic equation that will result in cos(x) = something.

OR - a way that does not require so much algebra but a little more knowledge of trigonometry.